A conventional mechanical construction level includes a vial partially filled with liquid so as to form an air or gas bubble in the vial. The vial is mounted in an elongated rail that is adapted to be positioned adjacent to and aligned with a surface whose orientation is to be sensed. The vial includes index marks positioned such that when the bubble is centered between the index marks, the rail is horizontal.
A number of efforts have been made to produce electronic leveling devices that are generally similar to mechanical levels but that use electrical and optical means to determine that the bubble is centered. In one prior arrangement, the vial has a rectangular cross section, and the bubble position is sensed by a pair of LED-phototransistor combinations positioned such that light from an LED reaches its associated phototransistor by passing horizontally and transversely through the upper portion of the vial. In such a device, light passes either through the bubble or through the liquid, depending upon the bubble position. By comparing the output of each phototransistor to a predetermined reference level, a determination is made as to whether or not the bubble is centered and the device is horizontal.
Although electronic level sensing devices possess a number of advantages when compared to conventional mechanical levels, prior electronic leveling devices have not achieved their full potential. Perhaps the principal limitation of most prior electronic devices is that they detect only the longitudinal ends of the bubble, and therefore do not provide an indication of the degree of deviation of the device from the horizontal. Some prior devices do not even indicate the direction of deviation. Prior devices have also typically made use of comparatively inefficient electronic circuits, which has resulted in excessive weight, bulk and power consumption. Certain prior devices have also used temperature sensitive circuits, and the accuracy of these devices has therefore been limited.